1. Field of the Invention
The present invention relates to an organic electroluminescent display device using an organic electroluminescent device (hereinafter referred to as an organic EL device) as a light emitting device.
2. Description of the Prior Art
An organic EL device has been expected as a new self light emitting device. Examples of the structure of the organic EL device include generally a structure (an SH-A structure) in which a hole transporting layer and a light emitting layer are formed between a hole injection electrode serving as an anode and an electron injection electrode serving as a cathode, a two-layer structure (an SH-B structure) in which a light emitting layer and an electron transporting layer are formed between a hole injection electrode and an electron injection electrode, and a three-layer structure (a DH structure) in which a hole transporting layer, a light emitting layer and an electron transporting layer are formed between a hole injection electrode and an electron injection electrode.
An electrode material having a large work function, for example, gold or ITO (an indium-tin oxide) is used as the hole injection electrode serving as an anode, while an electrode material having a small work function, for example, Mg is used as the electron injection electrode serving as a cathode.
Organic materials are used for the hole transporting layer, the light emitting layer and the electron transporting layer. A material having the property of a p-type semiconductor and a material having the property of an n-type semiconductor are respectively used for the hole transporting layer and the electron transporting layer. For the light emitting layer, a material having the property of an n-type semiconductor, a material having the property of a p-type semiconductor, and a material having the property close to neutrality are respectively used in the SH-A structure, the SH-B structure, and the DH structure.
In any of the structures, the organic EL device emits light upon recombination of holes injected from the hole injection electrode (the anode) and electrons injected from the electron injection electrode (the cathode) in the interface between the light emitting layer and the hole (or electron) transporting layer and in the light emitting layer. Consequently, the organic EL device can emit light at a lower voltage, which is very hopeful as a future display device, as compared with an inorganic EL device having a collision excitation type light emitting mechanism as its light emitting mechanism.
A display device using this type of organic EL device is disclosed in U.S. Pat. No. 5,550,066. The organic EL display device uses a thin film transistor (TFT) provided on a glass substrate as an active matrix driving device, and the organic EL device is formed in an array shape on the glass substrate.
In the above-mentioned conventional organic EL display device, an island composed of polycrystalline silicon is formed on the glass substrate, and the active matrix driving device composed of the TFT is formed on the island. In the above-mentioned display device, however, it is necessary to separately prepare a peripheral driving integrated circuit (IC), including a shift register and a latch circuit, for driving the active matrix driving device, and connect a driving device of the organic EL device formed on the glass substrate and the peripheral driving integrated circuit by wires or flat cables using a mounting technique. Therefore, the fabrication of the display device is troublesome, and the size thereof is increased.
On the other hand, JP-A-09-114398 (Int.cl.G09F 9/30) discloses an organic EL display device in which a monocrystalline silicon semiconductor substrate is used as a substrate, an active matrix driving device is constituted by a MOS transistor composed of monocrystalline silicone, and an organic EL device is formed in an array shape on the semiconductor substrate. In the organic EL display device, the monocrystalline silicone semiconductor substrate is used as a substrate, so that a peripheral driving integrated circuit is easy to incorporate, thereby making it possible to eliminate the necessity of externally providing the driving integrated circuit.
However, the above-mentioned organic EL display device is so constructed that the organic EL device emits light from its surface on the side of a silicone substrate, so that a portion, corresponding to a light emitting portion of the organic EL device, of the substrate is subjected to etching, to form a light transmitting portion. Therefore, the number of etching steps for forming the light transmitting portion is increased. Further, the mechanical strength of the display device is significantly weakened because the substrate is thinned by etching, to form the light transmitting portion such that the organic EL device emits light.